The present invention relates in general to a pile driving method and in particular, to an improved pile driving method by which piles can be relatively easily driven into a hard rock bed through utilization of a vibration pile driver and jetting of a high pressure water in combination.
In the field of construction in general including civil engineering, building construction and so forth, conventionally various methods are known for driving into a ground such objects as foundation piles, pillars, sheathing boards and the like. For example, the following are known and have been most widely employed: (1) methods utilizing impulsive force of a falling weight (such as drop hammering method, diesel pile hammering, and steam or compression air hammering), (2) a method utilizing vibration impulse (vibro-hammering method), (3) earth auger methods and (4) each of the above methods (1) to (3) incorporating an auxiliary method.
The pile driving is subject to a great influence with respect to the range of its possible application depending on the particular nature and, more particularly, the particular hardness of the ground for pile driving. The particular hardness may normally be represented in terms of the N value, which is shown by the number of hammerings required in a standard penetration test. In the standard test, a hammer of 63.5 kg in weight is freely dropped from 75 cm above, and the number of hammers is counted until the test sample hammered into has been penetrated to the depth of 30 cm. An N value of a higher value means a higher penetration resistance of ground and accordingly a harder ground condition.
In the instance of the current methods (1) above, the range of their possible application is limited to such a ground condition as having an N value below 50, and in case the piling ground has a greater N value, it can occur that the pile hammered comes out of an initially formed portion of the hall due to repellency of the hard ground or that the pile top becomes bent or otherwise damaged, whereby it often is completely infeasible to drive a pile.
With the vibro-hammering method, above (2), it comprises driving a pile as relied on vertical vibration applied at the head of the pile, and it has once drawn a wide attention in this field of the art for its advantage that it can be operated with less noise generation. However, its applicability is limited only to such ground conditions as having an N value not greater than 30, and as a method for pile driving under hard ground conditions, it fails to come up even to methods (1) above considered.
Then, with the above methods (3), they are also called drilling methods, according to which piles are driven to rotate and penetrated into a ground through their own rotation. The range of possible applicability in the case of methods (3) is further limited, and in terms of the N value, it is only within a range of 20--30.
The auxiliary method incorporated in most cases of the above (4) methods comprises jetting a relatively great amount of water under a relatively high pressure through a pipe fixed to a pile to be driven and thereby softening the ground in the vicinity of the pile top to facilitate the pile driving. In an actual example of the above vibro-hammering method (2) incorporated with such water jetting method, the pile driving is operated with use of a water supplied in an amount within a range of 1,000-2,000 l/min. and jetted under a pressure of from 100 to 200 kg/cm.sup.2, and the limitation relative to the N value is alleviated in this case up to a value on the order of 40 to 50. Further, in the case of the drilling method (3) above, the applicability is widened to possibly cover even such ground conditions as having an N value up to about 50, by incorporating thereto a high-pressure water jetting within a same range as mentioned above of the amount and pressure of the water.
However, in accordance with any of the existing methods above referred to, it is virtually impossible to satisfactorily operate a pile driving into a harder bed and, more specifically, rock bed having an N value exceeding 50, and in often cases the operation cannot have a pile effectively driven into rock or, otherwise, results in breakage of the top of the pile. In the case of driving piles into a rock bed, one of the above methods (4) is generally employed, and normally in this case the water guide pipe is immovably fixed to the pile so as to move up and down in accordance with the movement of the pile being driven. If a high pressure water is jetted in such a great amount and under such a high pressure as to be within the above-mentioned ranges, through a guide pipe mounted in the above manner, the counteraction of the water jetting (similar to a rocket) can be of a considerable degree. Consequently the pile is subject to an accordingly considerable force application in the direction opposite of the pile driving direction. Thus, the pile driving force is greatly lessened, whereby it is made difficult to effectively drive piles into a rock bed as before mentioned.
As pointed out above, with each of the typical methods (1) through (4) of a wide employment today, a limitation is unavoidable that the ground for piling is to have an N value smaller than about 50, and in the circumstances it has been widely regarded that pile driving into a hard ground having and N value above 50, is completely infeasible. Then, in certain cases it is proposed to operate for example boring of a hard rock bed, followed by placing of a pile into the bore and driving thereof. However, the operation relied on such method is highly complex and more so when piling is to be made into an underwater rock bed.
Accordingly, there has been a strong demand for providing a method by which construction piles can be directly driven into a hard rock bed.